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This quirky-looking iPhone 4 case by USB Fever has one unique feature most other cases lack: a dual SIM card slot. For $30 you get a case, room for an additional SIM card, and a dual-SIM adapter. Once you have your piggy-backed SIM card attached, you can toggle between the two cards using the iPhone’s settings menu. Unfortunately you can’t use both SIM cards simultaneously, and when you switch from one SIM card to the other, it takes a few minutes for your iPhone to search for a signal. During this time you may get a “No SIM Card Installed” warning, but this will disappear after a minute or two. Not a deal-breaker compared to needing two phones on a long trip.

This case isn’t the most attractive case out there (by far), but what it lacks in looks it makes up for in utility. The hassle of using more than one phone while traveling makes this case one to consider if you think you’ll be traveling internationally. Another bonus: no iPhone death grip problems, but all cases can of course make that claim.

I saw another product on the Simore website. Not exaxtly the same but the main advantage is that you can use 2 SIM cards (micro SIM and SIM standard format) and have two numbers active at the same time.
With this dual SIM card adapter you can make and receive calls on both SIM cards simultaneously with your iPhone4.
For the price and how to buy it, you have all the information on their website :Dual SIM Card - Mobile Dual SIM

I have had it on my 4S for about a half a year, and it works like a charm! Getting it set up at first is a bit of a pain, but patience is needed, (didn't have much of it at the time), but i have used it in USA, Germany, Italy, Netherlands, and France. Yes, i do have a prepaid chip for each country. I love it. I do NOT have a CDMA iphone.
Phone obviously does have to be JB!
Any questions, please ask.
bill